1. Field of the Invention
This invention relates to a wiring panel constructed with laminated layers of an insulating member composed of an organic substance and an electrically conductive member composed of copper as the principal constituent, and also to a method of producing such wiring panel.
2. Description of Prior Arts
In recent years, great demand is imposed on electronic apparatuses and appliances for their densification, miniaturization, and high speed in their operations. For this purpose, wiring panels which constitute an integral part of such electronic apparatuses and appliances are also required to have high density wiring as well as high speed signal transmission capability. In order to meet these requirements, attention has been drawn to copper or copper alloys having low resistivity, as the electrically conductive material, while attention has also been paid on polyimide resin which is capable of high density patterning, has high heat resistance, and has a low dielectric constant, as the insulating material.
Conventional wiring panel such as, for example, one which is described in "Denshi Joho Tsushin Gakkai Ronbun Shi--C (Journal of Electronic Information Communication Society of Japan--C), Vol. J71-C, No. 11 (1988), pp. 1510-1515" is of a construction as shown in FIG. 3 of the accompanying drawing (which is a cross-sectional view), wherein a reference numeral 1 designates a substrate, a reference numeral 4 denotes an electrically conductive layer comprising copper, and a numeral 3 refers to an insulating layer comprising polyimide.
This wiring panel made up of the substrate, copper, and polyimide is manufactured in the following process steps. First of all, a film of high purity copper of 99.99% or higher is formed on the substrate 1 to a predetermined film thickness by the evaporative deposition method. Subsequently, varnish containing therein polyamic acid, which is a precursor of polyimide, is coated over the copper film to a predetermined film thickness, and then the combination is subjected to heat-treatment at a temperature of about 350.degree. C. in a nitrogen atmosphere to change polyamic acid into polyimide, thereby forming the polyimide film. By sequential repetition of the copper film forming step and the polyimide film forming step, multi-layers are formed on the substrate, during which process appropriate patterning is effected on the copper film and the polyimide film to thereby enable a desired wiring panel to be manufactured.
By the way, the wiring panel, particularly the one for use in a high speed computer which is required to transmit high speed signals as they are being kept at high quality, is demanded to have low resistivity (.OMEGA..cm) of the conductive material, low dielectric constant of the insulating material, and further heat-resistance against various heat-treatments. As the conductive materials of low resistivity, there may be exemplified silver, copper, and others. Of these various species, copper is a material next to silver in its low resistivity, and it has electro-chemical stability comparable with that of silver, hence it is a material difficult to cause deterioration in its insulation among the wirings. Of various insulating materials, polyimide is one of those small numbers of organic materials having heat-resistance of as high as 300.degree. C. or above and low dielectric constant. On account of this, there has been conducted positive development of the wiring panel made up of copper and polyimide in combination, as in the above-mentioned example.
In the formation of a film of polyimide resin as the insulating layer, it has been a general practice to coat on the substrate a polyamic acid precursor which is soluble in a solvent, and then subject the precursor on the substrate to heat-treatment, thereby converting the polyamic acid precursor to polyimide resin.
There has, however, been a problem such that, since copper and polyamic acid precursor are highly reactive each other, and, in particular, since copper diffuses into polyimide resin at a high temperature condition, the reaction tends to occur readily between copper and polyimide resin with the consequence that polyimide resin decreases its heat-resistance. Further, depending on the state of reaction, there takes place peeling at the interface between copper and polyimide resin with the consequent problem of the combination not functioning as the wiring panel. This reaction between copper and polyimide resin is considered due to decomposition of the imide ring in the polyimide resin.
With a view to solving such point of problem, there have heretofore been devised a wiring panel, as disclosed in a thesis of F. S. Ohuchi, et al., "Journal of Vacuum Science and Technology", A6(3) (1988), pp. 1004-1006, wherein titanium, nickel, or other metal materials is interposed between copper and polyimide resin to thereby suppress diffusion of copper into polyimide. FIG. 4 of the accompanying drawing is a cross-sectional view schematically illustrating the wiring panel, in which a layer 6 of such other material is inserted between the copper layer 4 and the polyimide layer 3.
Furthermore, FIGS. 11 and 12 of the accompanying drawing respectively illustrate the sequential process steps of producing the conventional wiring panel, as disclosed, for example, in Japanese unexamined patent publication No. 151490/1984, which was devised for solving the above-mentioned disadvantages. FIG. 11 is a cross-sectional view of the wiring panel after formation of a different kind of metal film on the surface of the copper conductive wiring, while FIG. 12 is a cross-sectional view showing the wiring panel after patterning of the polyamic acid precursor film on the copper conductive wiring. In the drawing, a reference numeral 1 designates a ceramic substrate, a numeral 4 refers to a copper conductive wiring, a reference numeral 6 denotes a different kind of metal film (e.g., nickel film) as formed on the surface of the copper conductive wiring 4, and a numeral 3 refers to the polyamic acid precursor film as patterned on the copper conductive wiring 4.
This wiring panel is manufactured in such a manner that a different kind of metal film (e.g., nickel film 6) which does not bring about electron transfer between copper and the polyamic acid precursor is formed on the copper conductive wiring 4 (FIG. 11), and then the polyamic acid precursor film 3 is formed on the copper conductive wiring 4 (FIG. 12) to thereby suppress deterioration in the polyamic acid precursor film 3 and formation of a residual film at the time of the patterning.
However, since these other materials such as titanium, nickel, etc. are higher in their resistivity than copper, when they are used as mentioned above, the resistance of the wiring as a whole becomes higher. In addition, due to contact of the different kind of metal with the copper conductive wiring, a local electric field is generated to readily corrode the contacted part due to the battery effect, which inevitably brings about a problem such that the long-term operational reliability of the wiring panel as obtained becomes lowered.
Furthermore, titanium, nickel or other materials is interposed between copper or a copper alloy and polyimide for the purpose of physically separating them, on account of which the film thickness of such other material becomes unavoidably increased. In addition, since titanium or other material has higher resistivity than that of copper or copper alloys, there was also a problem of the electric resistance value of the wiring becoming higher with respect to the inter-layer connection, for example.